Heretofore mechanical means have been used in the production of single emulsions of the o/w (oil-in-water) type and of the w/o (water-in-oil) type, among others. More specifically, emulsions are generally produced by adding an emulsifying agent, such as a surfactant, and a liquid to be dispersed to a continuous phase liquid and stirring or frictionally mixing the resulting mixture by some mechanical means, such as a stirrer, homogenizer or colloid mill, to thereby comminute the dispersed phase. Further, the method of emulsification which comprises irradiating mixed liquids obtained in the above manner with ultrasonic waves to cause cavitation is also used.
However, the use of such mechanical means poses a problem that dispersed phase particles in the emulsion prepared (hereinafter sometimes referred to as emulsion particles) are considerably ununiform in size, so that the emulsion is poor in stability. In particular, when the dispersed phase concentration is high, a large amount of surfactant will be required for the improvement of emulsion stability.
Furthermore, it is difficult, by these known emulsion preparation methods, to suitably adjust the emulsion particle size depending on the intended use. Thus, for instance, while strict emulsion particle size control is very important in manufacturing monodisperse polymer microspheres, monodisperse inorganic microspheres and the like from emulsion particles, the prior art methods which use conventional mechanical means can hardly meet such requirement.
On the other hand, two methods are known for the production of double emulsions of the o/w/o or w/o/w type. One is the one-step emulsification method which utilizes phase inversion from w/o type emulsions to o/w type emulsions or from o/w type emulsions to w/o type emulsions and the other is the two-step emulsification method comprising dispersing with stirring a w/o or o/w type emulsion prepared in advance again in a continuous phase to obtain a w/o/w or o/w/o type emulsion. However, these methods have problems; for example, the yield of double emulsion particles may be low, and disruption of emulsion particles may occur, allowing a substance or substances added to flow out from the internal phase. Further, it is very difficult to obtain, by these methods, double emulsion particles size-controlled in the micron to submicron order, which are most important from the practical viewpoint.